Using nylon column adherence to fractionate lymphoid populations, it has been demonstrated that T cell-enriched and B cell-enriched populations could be separated from murine lymphoid preparations. By this methodology, T and non-T populations synergizing in the in vitro generation of cell-mediated cytotoxicity have been isolated. In another application of the same techniques, data has been generated suggesting that some T cell as well as B cell populations are capable of acting as stimulating cells in mixed lymphocyte reactions.